I just bought four Vetus shell ventilator Scirocco (Yup, the part number is "Scirocco") which are stainless, strong enough to walk on, and have a plastic base that helps keep water out and provides the attachment for the 3" hose. I have not had a chance to install them yet. About $120 for four.
I wouldn't put anything made of chrome-plated brass on my boat, not in Florida. I realize that fresh water is more forgiving, but stainless is stronger, too.
1975 32' Flybridge Sedan, twin Perkins 6-354 diesels, 1:1.53 velvetdrives, 16 X 19 props. Merritt Island, Florida